344 Association of American Geologists and Naturalists. 
how shall we derive from a vibration radiated from a deep focal spot or 
line, their remarkable number and isochronism? A simple vibratory 
jar sent through the crust, if competent to produce a great wave at all, 
could on the hypothesis produce no more than one, as the result of a single 
concussive force, so that when these waves follow each other, at regu- 
lar intervals of a quarter of a minute or more, for several minutes, 
we must admit that they are generated in some other manner, since 
there is no conceivable cause for a strictly isochronous repetition of the 
subterranean force. 
But an objection of another kind suggests itself, in the excessively 
fissured and crushed condition of the strata in many regions, and their 
extremely heterogeneous composition, which must inevitably lead to a 
rapid dispersion or breaking up of all regular waves of vibration within 
the rocky mass. 
An eminent British geologist has suggested that the undulatory mo- 
tion in earthquakes, may be of the nature of the vibration in a stretched 
cord when it is struck; but Prof. Rogers and his brother find it difficult 
to imagine, if we deny the theory of a pulsating fluid under the crust, how 
in a mass so little homogeneous and so unelastic, nodal vibrations could 
take place in the solid fabric of the globe, causing waves of the height 
and amplitude of the earthquake undulations. 
Rejecting the opinion that the vibratory jar is the cause of the undu- 
latory motion, they deem it more in accordance with known phenome- 
na, to recognize it as the effect, and to attribute the tremor to an exten- 
sive, minute fissuring and grinding together of the strata under the alter- 
nate dilatation and compression going on in every part of the rocky 
mass, during the undulation. 
The dimensions of the individual inundations would appear to be sus- 
ceptible in certain cases of direct calculation. Though the waves or 
temporary flexures must be of various magnitudes in different earth- 
quakes, their amplitude in the more violent convulsions is manifestly 
very great. Thus taking the data furnished by Darwin in his account 
of the earthquake of Conception, it may be shown that the probable 
width of each pulsation in that instance, was at least ten geographical 
miles, while there is reason to conclude that in the great Lisbon earth- 
quake, each wave of the crust had an amplitude amounting to twenty 
five miles. ° 
That the wave-like motion of the earth is of the character of an actu- 
al billowy pulsation, is shown not only by the visible heaving of the 
ground, but by the sensations produced, and by the alternate opening 
and closing of enormous parallel chasms of great depth, and the direc- 
tion of these, which is perpendicular to the course of the undulation. 
Of the manner in which the wave-like movement in earthquakes may 
